rebdoniel:
1. Many frum Jews are speaking Yiddish, not Hebrew. Various vowel and consonant shifts occured, as they have in German.
2. What defines proper Hebrew? Certainly it requires careful differentiation between ? and ?, and respect for the dagesh in all letters where it occurs (including daled and gimmel). The accent of Jews from Yemen might be the closest. The multiple “S” and “T” letters are all mistakes (natural ones, as many language evolve along similar lines) — note how all American mispronounce the gutteral “gh” as in how we would have written Ghanukah had there been any English-speaking Jews in the middle age.
3. Had the Jewish people been wiped out, Hebrew would have been frozen and there would be a “right” pronounciation. But we surived and have been writing, and speaking, Hebrew for millenia, and like all languages, it evolves.
4. Obviously the correct pronounciation is the way Ha-Shem spoke to us at Sinai. If the correct pronounciation was of any halachic significance, I’m sure that Torah would have been given on CDs so the matter would be clear.